Turbochargers have been designed which incorporate a compressor inlet structure that has become known as a "map width enhanced" (MWE) structure. Such an MWE structure is described in for example U.S. Pat. No. 4,930,979. In such arrangements, the compressor inlet comprises two coaxial tubular inlet sections, the inner inlet section being shorter than the outer section and having an inner surface which is an extension of a surface of an inner wall of the compressor housing which faces vanes defined by an impeller wheel mounted within the housing. An annular flow path is defined between the two tubular inlet sections, the annular flow path being open at the upstream end and opening at the downstream end through apertures communicating with the inner surface of the housing which faces the impeller wheel.
With an MWE inlet structure, when the flow rate through the compressor is high, air passes axially along the flow path defined between the two tubular sections towards the compressor wheel. When the flow through the compressor is low, the direction of air flow through the flow path is reversed so that air passes from the apertures adjacent the impeller wheel to the upstream end of the inner tubular section of the inlet structure. As is well known, the provision of such a flow path stabilises the performance of the compressor.
It is well known that compressors incorporating MWE inlet structures tend to exhibit higher levels of noise than conventional structures in which an inlet is defined by a single tubular member. This problem is addressed in British patent number 2256460 which disloses an MWE inlet which incorporates a noise-reduction baffle located upstream of the inner tubular section of the structure and retained within the upstream end of the outer tubular section of the structure. The baffle thus closes off the otherwise open axial end of the annular flow path defined between the inner and outer tubular sections of the inlet structure, the flow path communicating with the inlet through slots defined between the baffle and the upstream end of the inner tubular section of the inlet structure. The baffle may incorporate a conical section expanding outwards from the slots adjacent the upstream end of the inner tubular section of the structure.
The provision of a cone shaped baffle of the form illustrated in British patent 2256460 does reduce the noise emitted from the annular flow path defined between the two tubular sections of the structure and generally results in a reduction in the overall noise level. In some operational circumstances however the noise level within the main inlet flow passage is increased.
It is an object of the present invention to provide an improved MWE structure which addresses the noise problems referred to above.